Apparatus and method for gray field imaging

ABSTRACT

A beam of light is directed from a light source at a wafer on a chuck. The beam of light is reflected off the wafer toward a 2D imaging camera. Movable focus lenses in the path of the beam of light can independently change the illumination conjugate and the collection conjugate. A structured mask in an illumination path can be used and the beam of light can be directed through apertures in the structured mask. A gray field image of a wafer in a zone without direct illumination is generated using the 2D imaging camera and locations of defects on the wafer can be determined using the gray field image.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to the provisional patent applicationfiled Dec. 3, 2019 and assigned U.S. App. No. 62/943,170, the disclosureof which is hereby incorporated by reference.

FIELD OF THE DISCLOSURE

This disclosure relates to optical systems for semiconductor wafers.

BACKGROUND OF THE DISCLOSURE

Evolution of the semiconductor manufacturing industry is placing greaterdemands on yield management and, in particular, on metrology andinspection systems. Critical dimensions continue to shrink, yet theindustry needs to decrease time for achieving high-yield, high-valueproduction. Minimizing the total time from detecting a yield problem tofixing it determines the return-on-investment for a semiconductormanufacturer.

Fabricating semiconductor devices, such as logic and memory devices,typically includes processing a semiconductor wafer using a large numberof fabrication processes to form various features and multiple levels ofthe semiconductor devices. For example, lithography is a semiconductorfabrication process that involves transferring a pattern from a reticleto a photoresist arranged on a semiconductor wafer. Additional examplesof semiconductor fabrication processes include, but are not limited to,chemical-mechanical polishing (CMP), etch, deposition, and ionimplantation. An arrangement of multiple semiconductor devicesfabricated on a single semiconductor wafer may be separated intoindividual semiconductor devices.

Inspection processes are used at various steps during semiconductormanufacturing to detect defects on wafers to promote higher yield in themanufacturing process and, thus, higher profits. Inspection has alwaysbeen an important part of fabricating semiconductor devices such asintegrated circuits (ICs). However, as the dimensions of semiconductordevices decrease, inspection becomes even more important to thesuccessful manufacture of acceptable semiconductor devices becausesmaller defects can cause the devices to fail. For instance, as thedimensions of semiconductor devices decrease, detection of defects ofdecreasing size has become necessary because even relatively smalldefects may cause unwanted yield loss in the semiconductor devices.

However, detecting defects in 3D wafer structures and other advancedsemiconductor designs poses challenges. For optical inspection, a brightfield (BF) mode results in a strong reflection from a top surface of thewafer. The defect signal can be overwhelmed or limited by a bright waferpattern. A dark field (DF) mode typically suppressed all wafer patternsin a regular array region, but not for a random pattern because waferpattern information is lost for nuisance filtering. Without contrastinformation, it can be difficult to differentiate defects from nuisance.

Therefore, improvements to inspection systems are needed.

BRIEF SUMMARY OF THE DISCLOSURE

A system is provided in a first embodiment. The system includes a lightsource that generates a beam of light, an objective, a chuck configuredto hold a wafer in the path of the beam of light that passes through theobjective, a relay lens disposed in a path of the beam of light betweenthe light source and the objective, a tunable illumination aperturedisposed in the path of the beam of light between the light source andrelay lens, a first tube lens disposed in the path of the beam of lightbetween the relay lens and the objective, a first movable focus lensdisposed in the path of the beam of light between the first tube lensand the relay lens, a second movable focus lens disposed in the path ofthe beam of light between the objective and a 2D imaging camera, asecond tube lens disposed in the path of the beam of light between thesecond movable focus lens and the objective, and the 2D imaging cameraconfigured to receive light reflected from a wafer through theobjective. The first movable focus lens and the second movable focuslens are configured to be movable along the path of the beam of light toadjust an illumination conjugate between the light source and the waferand a collection conjugate between the wafer and the 2D imaging camera.The first movable focus lens and the second movable focus lens areconfigured to position an illumination focus at, above, or below asurface of the wafer. The 2D imaging camera is configured to generate agray field image of the wafer.

A structured mask can be disposed in the path of the beam of lightbetween the light source and the objective. The structured mask definesa plurality of apertures that the beam of light passes through. Aportion of the beam of light is blocked by the structured mask. Forexample, the structured mask can be disposed between the light sourceand the first movable focus lens or between the relay lens and the firstmovable focus lens.

The structured mask can be configured to tilt relative to the path ofthe beam of light.

An illumination numerical aperture can be from 0 to 0.9. A collectionpath numerical aperture can be at least 0.9.

The system can further include a processor in electronic communicationwith the 2D imaging camera. The processor is configured to identifydefects in the gray field image from the 2D imaging camera.

A method is provided in a second embodiment. The method includesdirecting a beam of light from a light source at a wafer on a chuck. Thewafer may include 3D structures. The beam of light is reflected off thewafer to a 2D imaging camera. A first movable focus lens and a secondmovable focus lens can be adjusted. The first movable focus lens isdisposed in a path of the beam of light between the light source and thewafer. The second movable focus lens is disposed between the wafer andthe 2D imaging camera. The adjusting includes independent changes to anillumination conjugate between the light source and the wafer and acollection conjugate between the wafer and the 2D imaging camera. Animage of the wafer is generated using the 2D imaging camera. The imageis a gray field image. A location of a defect on the wafer is determinedusing the image.

A focus of the beam of light can be below, at, or above a surface of thewafer. The focus of the beam of light can change in depth as the beam oflight scans across a surface of the wafer.

The method can further include directing the beam of light through astructured mask disposed in the path of the beam of light between thelight source and the first moveable relay lens. The structured maskdefines a plurality of apertures. The plurality of apertures form brightzones on a surface of the wafer and regions of the structured maskbetween the apertures form dark zones on the surface of the wafer.

An illumination numerical aperture can be from 0 to 0.9. A collectionpath numerical aperture can be at least 0.9.

The adjusting can include changing a position of the first movable focuslens such that the structure mask is focused and light in the brightzones leaks into the dark zones.

DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and objects of the disclosure,reference should be made to the following detailed description taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a diagram of a system in accordance with the presentdisclosure;

FIG. 2 is an image of an exemplary structured mask pattern (SMP);

FIG. 3 is an example of a gray field image using the structured maskpattern of FIG. 2;

FIG. 4 shows a relationship between illumination focus and collectionfocus; and

FIG. 5 is an embodiment of a flowchart of a method in accordance withthe present disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

Although claimed subject matter will be described in terms of certainembodiments, other embodiments, including embodiments that do notprovide all of the benefits and features set forth herein, are alsowithin the scope of this disclosure. Various structural, logical,process step, and electronic changes may be made without departing fromthe scope of the disclosure. Accordingly, the scope of the disclosure isdefined only by reference to the appended claims.

Advanced semiconductor devices tend to include 3D structures (DRAM, 3DNAND, Gate All-around). Defects below a device surface of a 3D structureare often hard to detect because the top surface's strong reflection cansaturate a sensor and because not enough light reaches the defectlocation, which can be hundred nanometers to a few microns below thesurface. A dark field imaging mode can mitigate the sensor saturationissue by filtering out specular reflection, but dark field imaging alsoloses the context for nuisance filtering. In addition, dark fieldimaging is not effective for random patterns.

Embodiments of the apparatus and method disclosed herein use a grayfield imaging mode, which is in between bright field and dark field.This imaging mode reduces specular reflection without eliminatingspecular reflection so that more light is allowed onto a device withoutsensor saturation. Hence, defect absolute signal is enhanced. The waferpattern remains and context-based or other advanced nuisance filteringalgorithms can be used.

In the embodiments disclosed herein, illumination optics with a highnumerical aperture (NA) may be used. Gray field imaging with highresolution for wafer field illumination can be achieved with or with astructured mask pattern. The illumination focus can be independentlyadjustable to at, above, or below the wafer surface. Collection focuscan be optimized for improved defect detection. The gray field imagingcan use a structured mask pattern with high resolution for wafer planeillumination, independently adjustable illumination and collectionfocus, or a combination thereof.

FIG. 1 is a diagram of a system 100. The structured mask pattern can beimaged onto a surface of the wafer 114 with demagnification. With highresolution and nearly diffraction limited illumination optics, astructured mask pattern can be projected onto a wafer plane withwell-defined sharp edges. In general, the system 100 is configured forgenerating optical-based output for a wafer 114 by directing light to(or scanning light over) and detecting light from the wafer 114. While awafer 114 is disclosed, a reticle or other workpiece also can be imaged.

Center illumination is illustrated in FIG. 1, but ring-shapedillumination is possible. The aperture shape can vary. An illuminationnumerical aperture can vary, such as from 0 to 0.9. A collection pathnumerical aperture can vary, and may be at least 0.9.

A light source 101 generates a beam of light 104. The light source 101is configured to direct the light to the wafer 114 at one or more anglesof incidence, which may include one or more oblique angles and/or one ormore normal angles. Conceptually, a gray field can be used for anywavelength range. The optimal wavelength typically depends on waferstack materials. For instance, if a wafer 3D stack includes substantialamounts of silicon or polysilicon, near-infrared wavelength may be used.In another example, if the wafer material is transparent, then visiblewavelengths or deep ultraviolet (DUV) wavelengths may be used for betteroptical resolution.

In an instance, the light source 101 may include a broad band plasma(BBP) source. In this manner, the light generated by the light source101 and directed to the wafer 114 may include broadband light. However,the light source 101 may include any other suitable light source such asa laser or light-emitting diode (LED). The laser or LED may include anysuitable forms known in the art and the light source 101 may beconfigured to generate light at any suitable wavelength or wavelengthsknown in the art. In addition, the light source 101 may be configured togenerate light that is monochromatic or nearly-monochromatic. In thismanner, the light source 101 may be a narrowband laser. The light source101 also may include a polychromatic light source that generates lightat multiple discrete wavelengths or wavebands.

An objective 113 is positioned proximate a chuck 115 configured to holda wafer 114 in the path of the beam of light 104. The beam of light 104passes through the objective 113. For ease of illustration, arrowsindicate the path of the beam of light 104.

The system 100 may also include a scanning subsystem configured to causethe beam of light 104 to be scanned over the wafer 114. The scanningsubsystem may include any suitable mechanical and/or robotic assembly(that includes chuck 115) that can be configured to move the wafer 114such that the light can be scanned over the wafer 114. In addition, oralternatively, the system 110 may be configured such that one or moreoptical elements of the system 100 perform some scanning of the lightover the wafer 114. The light may be scanned over the wafer 114 in anysuitable fashion, such as in a serpentine-like path or in a spiral path.

An optional lens 105, tunable illumination aperture 120, relay lens 106,optional structured mask 107, first mirror 108, first movable focus lens109, first tube lens 110 (which illustrates three lenses, but othernumbers are possible), and second mirror 111 are disposed in the path ofthe beam of light 104 between the light source 101 and the objective113. The illumination path can be from the light source 101 to the wafer114. The illumination aperture 120 is disposed in the path of the beamof light 104 between the light source 101 and the relay lens 106 orbetween the optional lens 105 and the relay lens 106. The first tubelens 110 is disposed between the relay lens 106 and the objective 113.The structured mask 107 is disposed between the light source 101 and thefirst movable focus lens 109. The first movable focus lens 109 isdisposed between the first tube lens 110 and the relay lens 106.

The first movable focus lens 109 can be translated (such as using anactuator) along the path of the beam of light 104, which is illustratedwith an arrow. The first movable focus lens 109 can adjust to make thestructured mask 107 pattern focused relative to a surface of the wafer114. The motion relationship between first movable focus lens 109 andwafer 114 (e.g., the top surface of the wafer 114) can depend on animplemented optical design, such as illumination numerical aperture,wavelength, and magnification. For instance, for an embodiment the ratiocan be 1000:1. In another embodiment the ratio can be 500:1. Wafer 114motion range in the z direction is typically less than 15 μm. Acollection path from the wafer 114 to the camera plane of the 2D imagingcamera 102 can form an image of both the wafer 114 and the pattern ofthe structured mask 107 when it is present.

A third mirror 112, second tube lens 116, zoom lens 117, and secondmovable focus lens 118 are disposed in the path of the beam of light 104reflected from the wafer 114 between the objective 113 and the 2Dimaging camera 102. The second movable focus lens 118 can be translated(such as using an actuator) along the path of the beam of light 104,which is illustrated with an arrow. The second tube lens 116 is disposedin the path of the beam of light 104 between the second movable focuslens 118 and the objective 113. The motion relationship between thesecond moveable focus lens 116 and the wafer 114 (e.g., the top surfaceof the wafer 114) can depend on the implemented optical design form,magnification, numerical aperture, or other variables.

The system 110 may include a number of other refractive and/orreflective optical elements that in combination focus the light from theoptical element to the wafer 114. Thus, the system 110 may include anyother suitable optical elements (not shown). Examples of such opticalelements include, but are not limited to, polarizing component(s),spectral filter(s), spatial filter(s), reflective optical element(s),apodizer(s), beam splitter(s), aperture(s), and the like, which mayinclude any such suitable optical elements known in the art. Inaddition, the system 110 may be configured to alter one or more of theelements of the illumination subsystem based on the type of illuminationto be used for generating the optical based output. For example, anillumination aperture (pupil plane) can take an annular, circular, arc,half-moon, or slit section form at any location of the entireillumination pupil for a suitable illumination angle of incidencetarget.

The 2D imaging camera 102 is configured to receive light reflected fromthe wafer 114 through the objective 113. The 2D imaging camera 102 isconfigured to generate a gray field image of the wafer.

The 2D imaging camera 102 may be any suitable detector known in the art.For example, the 2D imaging camera 102 can be a charge coupled device(CCDs), time delay integration (TDI) camera, or any other suitabledetector known in the art. 2D imaging also can be achieved by scanningthe wafer 114 or by scanning a 1D or a point detector, which may includea photo diode array, photo-multiplier tube (PMT), or avalanche photodiode (APD). The 2D imaging camera 102 may also include non-imagingdetectors or imaging detectors.

The optional structured mask 107 is disposed in the path of the beam oflight 104 between the light source 101 and the objective 113. In aninstance, the structured mask 107 is disposed in the path of the beam oflight 104 between the relay lens 106 and the first movable focus lens109 or between the relay lens 106 and the first mirror 108. A conjugateimage of the structured mask pattern is formed on the wafer plane usingthe first moveable focus lens 109 to adjust a best focal plane distancefrom the top surface of the wafer 114.

The structured mask 107 defines apertures 119. These are shown in thetop view of the structured mask 107 in the inset of FIG. 1. Light fromthe beam of light 104 passes through the apertures 119. The regions ofthe structured mask 107 between the apertures 119 blocks the beam oflight 104. A portion of the beam of light 104 can be blocked by thestructured mask 107. The structured mask pattern can take a differentduty cycle or a different geometry than that illustrated for differentapplication cases. For example, the structured mask 107 can have acheckboard pattern.

A structured mask 107 can be formed by various methods. For instance, astructured mask 107 can be made of chrome-on-glass where the area withchrome pattern blocks light. The structured mask 107 also be cut out ofa sheet of metal, graphite, or other materials. The structured mask 107also can be achieved by optical interference or diffraction phenomena toachieve dark/bright illumination pattern.

A thickness of the feature to define the edge of the structured mask 107can affect results. For chrome-on-glass, the chrome pattern thicknessmay be less than 0.5 μm. For a piece of metal, the pattern thickness canbe 250 μm.

The distance between the illuminated regions formed using the structuredmask 107 also can affect imaging. The width of the apertures 119 may beminimized to affect the gray image, though different applications mayuse different mask configurations.

As shown in FIG. 1, a structured mask 107 pattern is placed at a planeconjugate to a wafer plane. The pattern geometry can be a 2D gratingwith half pitch transmitting light while the other half blocks light,though other pitch values are possible. Any duty cycle can be used.

One example of a structured mask pattern is shown in FIG. 2. Thispattern is illuminated by a light source and an illumination aperturelimits an angle of incidence onto the structured mask. The opticalsystem between the structured mask 107 and the wafer plane images thestructured mask pattern onto the wafer plane with demagnification. Whenusing a numerical aperture (e.g., 0.9) with nearly diffraction limitedoptical performance, the structured mask pattern forms a sharp image atwafer plane with one or more well-defined edges. A well-defined edge canform a clear boundary between a directly-illuminated area and a grayfield imaging area. For wafer inspection, a gray field imaging area candefine an inspection care area. A sharp boundary can void direct lightcreeping into the intended gray field imaging area and can ensureuniform sensitivity in the care area. The wafer pattern can be modulatedby the structured mask pattern brightness across a field of view. Byadjusting the first movable focus lens 109, the structured mask 107 canbe focused at, above, or below a surface of the wafer 114 (e.g., in thez direction of FIG. 1) depending on a defect depth inside a wafer stack.A wafer image is collected through collection optics onto the 2D imagingcamera 102. By adjusting the second movable focus lens 118, the 2Dimaging camera 102 can capture a wafer pattern image at, above, or belowa surface of the wafer 114. Whether the focus is at, above, or below thesurface of the wafer can depend on the type or wafer of imagingapplication.

Using an actuator (not illustrated), the structured mask 107 can betranslated into and out of the beam of light 104 and/or tilted relativeto the beam of light 104 as shown in FIG. 1.

Turning back to FIG. 1, a processor 103 is in electronic communicationwith the 2D imaging camera 102. The processor 103 is configured toidentify defects in the gray field image from the 2D imaging camera 102.

FIG. 3 is an example of a gray field image using the structured mask ofFIG. 2 when the wafer is a mirror surface. The structured mask patternof FIG. 2 is a 2D binary grating. Light is blocked in a dark zone andtransmitted in a bright zone. The structured mask pattern in FIG. 3modulates the wafer pattern brightness, which are the thin verticalbright lines. The wafer pattern has a 3D structure. The dark zone fromFIG. 2 becomes a gray zone and black dots (defects) are apparent. It ismore difficult to distinguish black dots in the bright zone. The brightzones have no observable defect signal in FIG. 3, which may beoverwhelmed by strong specular reflection from wafer surface.

In FIG. 3, light in the bright zone can leak into the dark zone for a 3Dwafer structure through diffraction, secondary reflection from a bottomsurface, or other optical interactions inside a stack. The amount oflight that leaks can depend on the wafer, the illumination wavelength,the shape and dimension of the apertures in the structured mask, orother parameters. Dark zones appear as gray and appear as if they wereback-lit. Thus, the structured mask regions between the apertures becomegray. With light leaking into a dark zone from a bright zone, there maybe no specular light and the defect signal becomes apparent. The grayfield imaging mechanism can be achieved by directly illuminating areanext to region of interest for defect detection. A wafer's 3D structurecan make the region of interest appear as gray. Defects buried inside astack or close to a stack surface can become more apparent than adirectly-illuminated area.

The wafer 114 can be scanned with respect to the beam of light 104 sothat some or all of a surface of the wafer 114 is imaged. This allowsdefects to be captured across the surface of the wafer 114.

Illumination focus and collection focus can be adjusted to provide grayfield imaging with or without a structured mask. FIG. 4 shows arelationship between illumination focus and collection focus for theimage shown on the bottom.

From right to left on the bottom wafer image, the illumination focusgradually moves into a wafer z stack (e.g., by tilting structured mask107 in FIG. 1). In the collection path of FIG. 1, both the wafer planeand camera plane are normal to optical axis. A boundary of thestructured mask pattern becomes fuzzier from right to left. Also notethat wafer pattern (vertical lines) sharpness and defect visibility atbright zone increases from right to left as illumination focus graduallymoves below wafer surface. If it were a conventional bright field floodillumination wafer inspection illumination scheme, wafer patternsharpness would have been uniform from left to right.

The range of angles used when tilting the structured mask 107 can dependon the application. In one example, the structured mask can be tilted sothat a defocus range is approximately 10-20 depth of focus, which may beequivalent to 0.3-0.5 degree at a wafer plane.

While the embodiment of FIG. 4 tilts the structured mask, the structuredmask 107 also can be removed from the path of the beam of light 104.

This arrangement allows for independent focus adjustment of theillumination conjugate (structured mask to wafer) and collectionconjugate (wafer to 2D imaging camera). When using a high numericalaperture and diffraction-limited illumination optics, the illuminationpattern on the wafer has a narrow depth of focus and, consequently, theillumination light intensity z distribution inside a wafer can benarrower compared to a conventional non-diffraction limited illuminationoptics. This will help confine the illumination light intensity zdistribution of the wafer stack being illuminated. By shifting anillumination focus below a wafer surface, the intensity of surfacereflection can be reduced. The image path focus can be independentlyadjusted to achieve either improved pattern sharpness or improved defectsignal. FIG. 4 shows that illumination focus gradually moves below wafersurface from right to left (by tilting the structured mask pattern).

Wafer plane and camera plane may not be normal to optical axis. Usingtraditional illumination where optical performance is not diffractionlimited, the whole image plane should have the same wafer patternsharpness. However, in this image, only one X position forms a sharpfocus at camera. In other words, z position of the illumination focuscan modulate image sharpness. By adjusting illumination focus below awafer surface and imaging a path focus at a wafer surface, specularreflection from surface pattern can be reduced and a defect near thesurface can become more apparent.

In an instance, by adjusting illumination optics' numerical aperture,size, shape, and/or polarization, the gray zone's back-lit z-depth canbe optimized. The field distribution inside a wafer stack can change.Hence, defects at different depth can be selectively enhanced.

The wavelength, illumination angle, or other parameters of the light canvary depending on the material of the wafer 114. For example, thewavelength may be changed for polysilicon structures compared tocrystalline silicon or silicon oxide. Illumination angle may affectcertain materials more than wavelength in some instances.

Turning back to FIG. 1, the system 100 is provided herein to generallyillustrate a configuration that may be included in the systemembodiments described herein or that may generate optical based outputthat is used by the system embodiments described herein. The system 100configuration described herein may be altered to optimize theperformance of the system 100 as is normally performed when designing acommercial output acquisition system. In addition, the systems describedherein may be implemented using an existing system (e.g., by addingfunctionality described herein to an existing system). For some suchsystems, the methods described herein may be provided as optionalfunctionality of the system (e.g., in addition to other functionality ofthe system). Alternatively, the system described herein may be designedas a completely new system.

The processor 103 may be coupled to the components of the system 100 inany suitable manner (e.g., via one or more transmission media, which mayinclude wired and/or wireless transmission media) such that theprocessor 103 can receive output. The processor 103 may be configured toperform a number of functions using the output. The system 100 canreceive instructions or other information from the processor 103. Theprocessor 103 and/or an electronic data storage unit optionally may bein electronic communication with a wafer inspection tool, a wafermetrology tool, or a wafer review tool (not illustrated) to receiveadditional information or send instructions. For example, the processor103 and/or the electronic data storage unit can be in electroniccommunication with a scanning electron microscope (SEM).

The processor 103, other system(s), or other subsystem(s) describedherein may be part of various systems, including a personal computersystem, image computer, mainframe computer system, workstation, networkappliance, internet appliance, or other device. The subsystem(s) orsystem(s) may also include any suitable processor known in the art, suchas a parallel processor. In addition, the subsystem(s) or system(s) mayinclude a platform with high-speed processing and software, either as astandalone or a networked tool.

The processor 103 may be implemented in practice by any combination ofhardware, software, and firmware. Also, its functions as describedherein may be performed by one unit, or divided up among differentcomponents, each of which may be implemented in turn by any combinationof hardware, software, and firmware. Program code or instructions forthe processor 103 to implement various methods and functions may bestored in readable storage media, such as a memory in the electronicdata storage unit or other memory.

If the system 100 includes more than one processor 103, then thedifferent subsystems may be coupled to each other such that images,data, information, instructions, etc. can be sent between thesubsystems. For example, one subsystem may be coupled to additionalsubsystem(s) by any suitable transmission media, which may include anysuitable wired and/or wireless transmission media known in the art. Twoor more of such subsystems may also be effectively coupled by a sharedcomputer-readable storage medium (not shown).

The processor 103 may be configured to perform a number of functionsusing the output of the system 100 or other output. For instance, theprocessor 103 may be configured to send the output to an electronic datastorage unit or another storage medium. The processor 103 may be furtherconfigured as described herein.

The processor 103 may be configured according to any of the embodimentsdescribed herein. The processor 103 also may be configured to performother functions or additional steps using the output of the system 100or using images or data from other sources.

Various steps, functions, and/or operations of system 100 and themethods disclosed herein are carried out by one or more of thefollowing: electronic circuits, logic gates, multiplexers, programmablelogic devices, ASICs, analog or digital controls/switches,microcontrollers, or computing systems. Program instructionsimplementing methods such as those described herein may be transmittedover or stored on carrier medium. The carrier medium may include astorage medium such as a read-only memory, a random access memory, amagnetic or optical disk, a non-volatile memory, a solid state memory, amagnetic tape, and the like. A carrier medium may include a transmissionmedium such as a wire, cable, or wireless transmission link. Forinstance, the various steps described throughout the present disclosuremay be carried out by a single processor 103 or, alternatively, multipleprocessors 103. Moreover, different sub-systems of the system 100 mayinclude one or more computing or logic systems. Therefore, the abovedescription should not be interpreted as a limitation on the presentdisclosure but merely an illustration.

FIG. 5 is an embodiment of a flowchart of a method 200. A beam of lightis directed from a light source to a wafer on a chuck at 201. The beamof light is reflected off the wafer toward a 2D imaging camera at 202.The focus of the beam of light can be below the surface of the wafer,but also can be above or at the surface of the wafer. The focus canchange in depth as the beam of light scans across the surface of thewafer.

A first movable focus lens and a second movable focus lens can beadjusted at 203. The first movable focus lens is disposed in a path ofthe beam of light between the light source and the wafer. The secondmovable focus lens is disposed between the wafer and the 2D imagingcamera. The adjusting includes independent changes to an illuminationconjugate and a collection conjugate.

The adjusting can include changing a position of the first movable focuslens such that the structure mask is focused and light in the brightzones leaks into the dark zones.

An image of the wafer is generated using the 2D imaging camera at 204.The image is a gray field image. A location of a defect on the waferusing the image is determined at 205. This determination can use aprocessor, such as the processor 103 of FIG. 1. The processor candetermine the location of a defect in the image using, for example,differences in the various pixels. The processor can compare a pixelagainst neighboring pixels to determine if a defect is present.

The beam of light can be directed through a structured mask disposed inthe path of the beam of light between the light source and the firstmoveable relay lens. The structured mask defines a plurality ofapertures. The apertures form bright zones on the surface of the waferand regions of the structured mask between the apertures form dark zoneson the surface of the wafer, which becomes a gray zone in a collectionpath after interacting with a 3D structure on the wafer.

Turning back to FIG. 1, the processor 103 is in communication with thesystem 100. The processor 103 can be configured to perform or sendinstructions for some or all of the steps of method 200.

An additional embodiment relates to a non-transitory computer-readablemedium storing program instructions executable on a controller forperforming a computer-implemented method, as disclosed herein. Anelectronic data storage unit or other storage medium may containnon-transitory computer-readable medium that includes programinstructions executable on the processor 103. The computer-implementedmethod may include any step(s) of any method(s) described herein,including method 200.

The program instructions may be implemented in any of various ways,including procedure-based techniques, component-based techniques, and/orobject-oriented techniques, among others. For example, the programinstructions may be implemented using ActiveX controls, C++ objects,JavaBeans, Microsoft Foundation Classes (MFC), Streaming SIMD Extension(SSE), or other technologies or methodologies, as desired.

Although the present disclosure has been described with respect to oneor more particular embodiments, it will be understood that otherembodiments of the present disclosure may be made without departing fromthe scope of the present disclosure. Hence, the present disclosure isdeemed limited only by the appended claims and the reasonableinterpretation thereof.

What is claimed is:
 1. A system comprising: a light source thatgenerates a beam of light; an objective; a chuck configured to hold awafer in a path of the beam of light that passes through the objective;a relay lens disposed in the path of the beam of light between the lightsource and the objective; a tunable illumination aperture disposed inthe path of the beam of light between the light source and the relaylens; a first tube lens disposed in the path of the beam of lightbetween the relay lens and the objective; a first movable focus lensdisposed in the path of the beam of light between the first tube lensand the relay lens; a 2D imaging camera configured to receive lightreflected from the wafer through the objective; a second movable focuslens disposed in the path of the beam of light between the objective andthe 2D imaging camera; and a second tube lens disposed in the path ofthe beam of light between the second movable focus lens and theobjective, wherein the first movable focus lens and the second movablefocus lens are configured to be movable along the path of the beam oflight to adjust an illumination conjugate between the light source andthe wafer and a collection conjugate between the wafer and the 2Dimaging camera, and wherein the first movable focus lens and the secondmovable focus lens are configured to position an illumination focus at,above, or below a surface of the wafer; and wherein the 2D imagingcamera is configured to generate a gray field image of the wafer.
 2. Thesystem of claim 1, further comprising a structured mask disposed in thepath of the beam of light between the light source and the objective,wherein the structured mask defines a plurality of apertures that thebeam of light passes through, and wherein a portion of the beam of lightis blocked by the structured mask.
 3. The system of claim 2, wherein thestructured mask is disposed between the light source and the firstmovable focus lens.
 4. The system of claim 3, wherein the structuredmask is disposed between the relay lens and the first movable focuslens.
 5. The system of claim 2, wherein the structured mask isconfigured to tilt relative to the path of the beam of light.
 6. Thesystem of claim 1, wherein an illumination numerical aperture of thesystem is from 0 to 0.9.
 7. The system of claim 1, wherein a collectionpath numerical aperture of the system is at least 0.9.
 8. The system ofclaim 1, further comprising a processor in electronic communication withthe 2D imaging camera, wherein the processor is configured to identifydefects in the gray field image from the 2D imaging camera.
 9. A methodcomprising: directing a beam of light from a light source at a wafer ona chuck; reflecting the beam of light off the wafer to a 2D imagingcamera; adjusting a first movable focus lens disposed in a path of thebeam of light between the light source and the wafer and a secondmovable focus lens disposed between the wafer and the 2D imaging camera,wherein the adjusting includes independent changes to an illuminationconjugate between the light source and the wafer and a collectionconjugate between the wafer and the 2D imaging camera; generating animage of the wafer using the 2D imaging camera, wherein the image is agray field image; and determining a location of a defect on the waferusing the image.
 10. The method of claim 9, wherein a focus of the beamof light is below a surface of the wafer.
 11. The method of claim 9,wherein a focus of the beam of light is at a surface of the wafer. 12.The method of claim 9, wherein a focus of the beam of light is above asurface of the wafer.
 13. The method of claim 9, wherein a focus of thebeam of light changes in depth as the beam of light scans across asurface of the wafer.
 14. The method of claim 9, further comprisingdirecting the beam of light through a structured mask disposed in thepath of the beam of light between the light source and the firstmoveable relay lens, wherein the structured mask defines a plurality ofapertures, and wherein the plurality of apertures form bright zones on asurface of the wafer and regions of the structured mask between theapertures form dark zones on the surface of the wafer.
 15. The method ofclaim 14, wherein the adjusting includes changing a position of thefirst movable focus lens such that the structured mask is focused andlight in the bright zones leaks into the dark zones.
 16. The method ofclaim 9, wherein an illumination numerical aperture is from 0 to 0.9.17. The method of claim 9, wherein a collection path numerical apertureis at least 0.9.
 18. The method of claim 9, wherein the wafer includes a3D structure.